Research On Large Transformer Transient Considering Zero Mode Inrush Current Under Sympathetic Interaction

In order to meet the growing demand for power supply in economic development and people’s lives,the scale and transmission capacity of my country’s power system continue to expand.In dense areas of the grid such as large power stations,power hubs and load centers,electrical equipment is increasingly tight-connected,and the complex electromagnetic coupling interaction characteristics presented raise new challenges to the safe operation of power equipment and reliable protection of power grid.Large transformers are the key equipment for power transmission and transformation,and are also indispensable equipment for large-scale access of new energy power generation,FACTS component connection,and ultra-high voltage DC transmission in the current new power system.At present,in addition to the ordinary transformer,acted as an important measure to limit the short-circuit current,high-impedance transformers of various types have also been widely used.In recent years,transformers and their associated power grid protection misoperation accidents have occurred frequently.These are mostly related to the structural type of the transformer(such as the high-impedance transformer),complex transient characteristics and their coupling characteristics with each other.As a result,academic and engineering circles pay close attention to the transient characteristics and interaction of transformer.From the engineering scene and accident phenomenon,it is necessary to construct a complete and accurate transformer inrush current and fault model to study its transient characteristics and interaction mechanism between transformers,and propose protection countermeasures.The research has important scientific significance and engineering value to ensure the safe operation of transformer and reliable protection of power system.In recent years,a large number of high-impedance transformers have been put into operation and they have changed the inrush current characteristics.There was a zero-mode inrush current with large amplitude and very slow attenuation when it was put into operation,which led to a serious accident of overstepping and malfunctioning of the long-delay protection of the upper line,and also brought greater risks to the safety of the transformer.At present,there is no reasonable explanation for generation mechanism of the inrush current,and there are no effective countermeasures.The main reason is that the existing transformer models cannot meet the needs of complex transient characteristics analysis of various transformers.This paper aims to improve the safe operation level and related protection performance of large-scale transformers under complex working conditions.From structural modeling,mechanism analysis,transient characteristics simulation,physical dynamic model experiments and on-site wave recording verification,etc.,the research on the transient characteristics and protection methods of large transformers considering zero-mode inrush current under sympathetic interaction is carried out.The main contents include:build a model reflecting the zero-mode inrush current characteristics of transformers with different structures under sympathetic interaction and an internal fault model taking into account inrush current,analyze the transient characteristics of zero-mode inrush current,reveal the mechanism of its slow attenuation and the cause of the associated protection malfunction,research improved protection methods for transformers and adjacent components and measures to suppress inrush current of transformers further.Aiming at the difficulty of constructing electromagnetic transient models of transformers with different structures,starting from the characteristics of magnetic circuit and basic electromagnetic differential equations,considering the structure of windings and iron cores,an electromagnetic transient model that can reflect the characteristics of transformers with different structures zero-mode inrush current under sympathetic interaction is proposed.Based on this,internal fault model considering inrush current is proposed.Parameter compensation method is used to solve the lack of modeling of transformers with winding arrangement structure in common simulation platforms.A physical dynamic model experimental platform is built to test the accuracy of the model.The results show that during the inrush current simulation,the simulation errors of the maximum inrush current peak value do not exceed 1%.The correctness and effectiveness of model have been verified,laying a foundation for the complex transient characteristics mechanism,simulation analysis and impact analysis on related protection of large transformers.Aiming at the problem of analyzing the characteristics of zero-mode inrush current under the interaction of transformers and its impact on protection,the equivalent circuit of zero-mode inrush current taking into account the sympathetic interaction and the arrangement of transformer windings is obtained through analysis.Based on circuit analysis,it is pointed out that sympathetic interaction may cause various types of zero-mode inrush current attenuation to be slower than when there is no sympathetic interaction,and it is revealed that the opposite polarity of the aperiodic components of the zero-mode equivalent potential of the operating transformer and the energized transformer is the mechanism for the slowing down of the zero-mode inrush current attenuation of the energized transformer,and the mechanism that the“complementary effect”of the two potentials leads to the slow attenuation of the zero-mode inrush current at the source side,which has passed the simulation verification.The simulation analysis shows that the zero-mode inrush current under sympathetic interaction will lead to an increase in the probability of misoperation of the zero-sequence protection of adjacent bus ties or lines of transformers with different structures,and may also cause the misoperation of its own differential protection.In order to prevent the protection malfunction of the transformer and its adjacent components caused by the zero-mode inrush current,the harmonic characteristics of the zero-mode inrush current are analyzed for the zero-sequence protection,and an inrush current restraint strategy based on the ratio of the second and third harmonics of the zero-mode inrush current is proposed.It solves the problem that the existing zero-sequence protection restraint strategy of the adjacent components of transformers with different structures may fail and malfunction due to the sympathetic interaction.Aiming at the differential protection of transformer,an inrush current restraint criterion based on the zero-mode inrush current correlation of the primary and secondary sides and the saturation characteristics of the iron core is proposed,which can be combined with the traditional second harmonic restraint criterion to realize the phase-by-phase inrush current restraint of the proportional differential protection.And it can adapt to Y,d-connected non-discrete transformer protection against inrush current malfunction.Compared with other restraint methods,the sensitivity and quickness are better under small turns ratio faults.The effectiveness of the above strategy is verified by simulation,dynamic model experiment and on-site wave recording.Aiming at the problem that the inrush current suppression effect is affected by the uncertain remanence of the transformer due to its complex shutdown conditions,an inrush current suppression strategy is proposed to generate saturation remanence in the iron core by applying an external DC current source to realize controllable remanence and closing phase,so that the minimum inrush current can be obtained through phase-splitting and time-division closing.And based on the magnetic circuit,a calculation model for magnetizing current of transformers with different structures is constructed.The model solution shows that the required magnetizing current is small.Taking into account the magnetic flux unbalance effect and other factors,the simulation shows that the inrush current can be suppressed to below 0.5 I_N,and the dispersion of the circuit breaker can be suppressed to below 0.9 I_N within(-2ms-2ms),and the effect is better than the existing phase-controlled inrush current suppression strategy,which can improve the safe operation level of the transformer.At the same time,the related protection action performance is improved.At the end of the paper,the main research results obtained are summarized,the main innovation points are extracted,and the future research work is prospected.